The present invention relates to a magneto-resistive device and a method of manufacturing the same, and a magnetic head, a head suspension assembly and a magnetic disk apparatus which use the magneto-resistive device.
With the trend to a larger capacity and a smaller size of hard disk drives (HDD), heads are required to have a higher sensitivity and a larger output. To meet these requirements, strenuous efforts have been made to improve the characteristics of GMR heads (Giant Magneto-Resistive Head) currently available on the market. On the other hand, intense development is under way for a tunnel magneto-resistive head (TMR head) which can be expected to have a resistance changing ratio twice or more higher than the GMR head.
Generally, the GMR head differs from the TMR head in the head structure due to a difference in a direction in which a sense current is fed. A head structure adapted to feed a sense current in parallel with a film surface, as in a general GMR head, is referred to as a CIP (Current In Plane) structure, while a head structure adapted to feed a sense current perpendicularly to a film surface, as in the TMR head, is referred to as a CPP (Current Perpendicular to Plane) structure. Since the CPP structure can use a magnetic shield itself as an electrode, it is essentially free from short-circuiting between the magnetic shield and a device (defective insulation) which is a serious problem in reducing a lead gap in the CIP structure. For this reason, the CPP structure is significantly advantageous in providing a higher recording density.
Other than the TMR head, also known as a head in CPP structure is, for example, a CPP-GMR head which has the CPP structure, though a spin valve film (including a specular type and dual spin valve type magnetic multilayer films) is used for a magneto-resistive device.
Any type of CPP-based heads has an upper electrode and a lower electrode for supplying a current to a magneto-resistive layer formed on a base, formed on the top (opposite to the base) and on the bottom (close to the base) of the magneto-resistive layer, respectively. The CPP-based head comprises an insulating layer for limiting a current path between the upper electrode and lower electrode is arranged around a main layer (for example, a tunnel barrier layer in a TMR head) of the magneto-resistive layer. The limited current path substantially matches an effective region for detecting a magnetic field from a magnetic recording medium. A TMR head is disclosed as an example of the CPP-based head in JP-A-2001-23131 corresponding to U.S. Pat. No. 6,473,257 and JP-A-2001-52316 corresponding to U.S. patent application Publication No. 2003/0151859.
In a conventional general CPP-based head as disclosed in JP-A-2001-23131, an insulating layer for limiting a current path between an upper electrode and a lower electrode is formed of a single-layer film. This insulating layer is generally made of Al2O3 or SiO2.
Generally, for manufacturing a conventional CPP-based head as disclosed in JP-A-2001-23131, constituent layers formed on a substrate, which make up a magneto-resistive layer, are milled using a resist mask to pattern the constituent layers. Then, the resist mask is used as it is to form an insulating layer of Al2O3 or SiO2 around the constituent layers by a lift-off method.
On the other hand, in a conventional TMR head disclosed in JP-A-2001-52316, an insulating layer for limiting a current path between an upper electrode and a lower electrode is composed of a first insulating layer formed near an end face of a ferromagnetic tunnel junction film (corresponding to a magneto-resistive layer) having a tunnel barrier layer as well as a pinned layer and a free layer which sandwich the tunnel barrier layer, and a second insulating layer which surrounds the end face of the ferromagnetic tunnel junction film through the first insulating layer. The first insulating layer is made of oxides of metal materials which constitute the ferromagnetic tunnel junction film pattern formed within the ferromagnetic tunnel junction film pattern. In other words, the first insulating layer is a layer made of metal oxides produced by oxidizing the constituent layers themselves of the ferromagnetic tunnel junction film, used as base materials, respectively, and is not a layer disposed on the end face of the ferromagnetic tunnel junction film pattern from the outside of the ferromagnetic tunnel junction film pattern. Consequently, metal oxide films made of different materials from one another are in contact with the end faces of a plurality of layers made of different materials from one another, which make up the ferromagnetic tunnel junction film, and the first insulating layer is composed of a sequence of these metal oxide films made of different materials from one another. The second insulating layer is made of an Al oxide, a Si oxide, or the like.
For manufacturing the conventional TMR head disclosed in JP-A-2001-52316, (a) a ferromagnetic tunnel junction film pattern (constituent layers which make up a magneto-resistive layer) formed on a substrate is milled using a resist mask to pattern the constituent layers; (b) the end face portions of the constituent layers themselves, used as base materials, are naturally oxidized or oxidized by a plasma oxidation method or the like to produce the first insulating layer from the end faces themselves; and (c) the second insulating layer is formed around the constituent layers using the resist mask as it is by a lift-off method.
According to the conventional TMR head disclosed in JP-A-2001-52316, even if the milling causes milling re-deposits to stick near the ends of the constituent layers during the manufacturing, the milling re-deposits are oxidized and included in the first insulating layer which does not provide a bypass path for a sense current, advantageously preventing a reduction in the MR ratio, as described in JP-A-52316.
It should be understood that generally, magnetic heads have not only a reproducing device such as a TMR device, a GMR device and the like, but also a recording device such as an inductive magnetic transducing device and the like, so that a composite magnetic head is typically provided for reproducing and recording magnetic information. During manufacturing of such a composite magnetic head, generally, a reproducing device is formed on a substrate before a recording device is laminated thereon. Then, annealing is performed as a photoresist curing step when a coil is fabricated during the fabrication of the recording device. For example, JP-A-2001-52316 describes that for manufacturing a composite magnetic head which has a recording device laminated on a TMR device, annealing is performed for two hours at 250° C. as a photoresist curing step during the fabrication of a coil of the recording device.
The result of a research made by the inventors has revealed that the conventional magnetic heads as disclosed in JP-A-2001-23131 and JP-A-2001-52316 suffer from deteriorated characteristics of the TMR devices due to the annealing. In this regard, description will be made below.
The inventors fabricated a magnetic head similar to that disclosed in JP-A-2001-23131. The fabricated magnetic head had an inductive magnetic transducing device laminated on a TMR device as a recording device. Also, annealing was performed as a photoresist curing step during the fabrication of a coil of the recording device. Further, in the course of the fabrication of the magnetic head, the fabricated TMR device underwent the first measurement of the characteristics thereof (the resistance and MR ratio of the TMR device) before the creation of the recording device on the fabricated TMR device. Then, the TMR device again underwent the second measurement of the characteristic thereof (the resistance and MR ratio of THE TMR device) after the recording device had been created.
A comparison of the results of the first measurement with the results of the second measurement has revealed that the characteristics of the TMR device after the creation of the recording device were significantly deteriorated as compared with those before the creation of the recording device, contrary to an assumption that the characteristics of the TMR device would be the same before and after the creation of the recording device. Specifically, the resistance of the TMR device taken in the second measurement was higher than the resistance of the TMR device taken in the first measurement, while the MR ratio of the TMR device taken in the second measurement was lower than the MR ratio of the TMR device taken in the first measurement. The TMR device has a challenge of reducing the resistance of the device itself because noise proportionally increases as the resistance of the device is higher. Further, a higher MR ratio is desired because a reduced MR ratio causes a smaller head output.
The results of more detailed experiments made by the inventors have revealed that the aforementioned deterioration in the characteristics of the TMR device (increased resistance and reduced MR ratio) are caused by the annealing performed for fabricating the recording device.
Magneto-resistive devices such as the TMR device have a variety of applications such as a magnetic detector, MRAM (Magnetic Random Access Memory), and the like, other than magnetic heads, and the annealing is sometimes involved in these applications.